Electric generators or alternators are known which employ two claw-pole rotors, mounted on a common rotor shaft driven from the engine of a vehicle. The claw-pole rotors, each of which forms a field system, have outer surfaces which are at any given moment opposite one of two armature lamination stacks. These alternators are cooled by a cooling air stream circulating through the housing thereof. See German Patent Disclosure Document DE-OS No. 31 29 817, corresponding to our co-pending U.S. application Ser. No. 388,891, filed June 16, 1982. This type of generator requires a high-cost construction as a result of the duplicate used and is therefore expensive to produce.